This invention relates to a method and apparatus for applying a cement mortar or concrete by projection, and more particularly by centrifugal force.
Application of a cement mortar or concrete by blasting can be done without preassembling a frame or mould, and is not necessary to disassembly the mold after setting of the applied mortar or cement so that surface finishing can be immediately made. Thus, this method not only greatly simplified the work but also greatly shortens the working period. For this reason, in present days, the method of blasting is used in many cases.
Among the methods of blasting may be mentioned, the wet method, dry method and semiwet method. However, these prior art methods are advantageous in one case but disadvantageous in other cases. More particularly, according to the blasting method, such a hydraulic substance as cement, an aggregate, and water are kneaded to prepare a green concrete having a desired water to cement ratio (W/C), the green concrete is conveyed to the working field with a concrete pump, and then projected upon a wall surface through a nozzle. With this method, since cement is sufficiently wetted the concrete product has a high mechanical strength. However, where a green concrete of a W/C ratio manifesting the highest strength is prepared, such green concrete is difficult to convey through a conduit because of its high frictional resistance and high viscosity. Consequently, a pump pressure of the order of 50 Kg/cm.sup.2 is necessary thus requiring a large driving power and strong conduit. Even with such expedience the maximum distance of conveyance is at most 50 to 60 m so that it is impossible to apply this method for constructing a long tunnel. Accordingly, in practice, the ratio W/C is increased more or less to improve fluidity of the green concrete, thus making it impossible to fully take advantage of the wet method. On the other hand, according to the dry method, concrete ingredients are conveyed to the field in a dry state by high pressure air, and the necessary quantity of water is added to the concrete ingredients in a blasting nozzle at the working field. The pressure of the air necessary to convey the concrete ingredients through a pipe may be about 5-6 Kg/cm.sup.2 so that the dry method is advantageous to convey the concrete ingrediates over a long distance with a simple and inexpensive installation. However, kneading of the dry ingredients and water can not be performed efficiently and cement dust is generated in a great quantity so that it is impossible to apply kneaded concrete over a long time. Moreover, the mechanical strength of the blasted concrete is only one half of that of the concrete formed by the wet method, which is a fatal defect of the dry method. The semiwet method is an intermediate method in which water is incorporated at an intermediate point of the conveyance pipe instead of at the end thereof. Where water is added at a point 5 to 6 m ahead of the nozzle, the fluidity decreases greatly thus clogging the pipe, so that it is impossible to use a long pipe as in the dry method. Where water is added at a point close to the nozzle, it is difficult to sufficiently knead the concrete, this difficulty being similar to that encountered in the dry method. With any method, the resulting concrete is blasted to a surface by high pressure air so that concrete once applied onto the surface tends to be blown away by the blast. A quick setting agent normally used at the time of blasting is added at the nozzle, but it is difficult to uniformly admix the quick setting agent with the concrete whereby the applied concrete lacks uniformity. Also loss of the quick setting agent at the time of blasting is high and the lost agent forms a stimulative atmosphere which makes it difficult to work over a long time. Considering the shear strength of the blasted concrete layer, in both of the dry method and the wet method the shear strength is low because in the former air quantity in the blasted layer is large, while in the latter, the content of water is high.